The invention relates to a device for searching the optimal sampling instant intended to retrieve a sample, referred to as optimal sample, from input samples and comprising an amplitude-computing member for computing the average amplitudes of the input samples.
The invention also relates to a receiver for a packet transmission system intended to receive data packets representing symbols transmitted by a terminal or a head station, the receiver comprising a sub-sampling device for generating a plurality of input samples from the received data.
The invention further relates to a packet transmission system comprising at least one terminal and a head station.
The invention finally relates to a method of recovering the optimal sampling instant intended to retrieve a sample, referred to as optimal sample, from input samples, comprising a step of computing the amplitude for determining the maximum of the average amplitudes of these samples and for deriving the optimal sample.
The invention finds important applications in the field of satellite or cable transmissions and notably in return path transmissions in which a plurality of terminals is capable of transmitting data packets to a head station in accordance with a time-division multiplex mechanism.
The article entitled xe2x80x9cDigital Communication Receivers Synchronization Channel Estimation and Signal Processingxe2x80x9d by H. Meyr, M. Moeneclay and S. Fechtel published in the Wiley Series editions in Telecommunications and Signal Processing describes on pages 283 to 289 a technique for computing the optimal sampling instant. In the majority of time-division transmission systems it is necessary to effect a sub-sampling at the receiver end so as to retrieve the sampling instant used at the transmitter end. The technique described in the quoted article recommends computation of the average energies of each sample for selecting the optimal sample having the maximum average energy. This technique is costly as regards the number of computations because it necessitates a computation of the average energy for all the samples before a decision on the optimal sample is made.
It is an object of the invention to provide a very rapid recovery of the optimal sampling instant by considerably limiting the number of computations and thus the operating costs. To this end, a device as described in the opening paragraph is characterized in that the computing member uses only one part of the input samples for determining the optimal sample having the maximum average amplitude among all the input samples. Thus, instead of computing the average energies of all the input samples, the computation is effected for several predetermined samples only.
In accordance with an important characteristic feature of the invention, the device for searching the optimal sampling instant comprises:
a device for taking predetermined input samples to be supplied to the computing member with a view to determining the sample having the maximum average amplitude, referred to as approximate maximum,
a selector for selecting a supplementary input sample comprised between the approximate maximum and the neighboring sample whose previously computed average amplitude is the largest and for supplying said sample to the computing member with a view to computing its average amplitude, and
a comparator for comparing the average amplitude of the selected sample with the previous approximate maximum and for deriving a new approximate maximum.
In accordance with a preferred embodiment of the invention, the supplementary sample is situated midway between the last approximate maximum and the neighboring sample whose previously computed average amplitude is the largest. This embodiment has the advantage that the precision of the searched maximum is improved by a factor of 2 upon each iteration of computing the amplitude, i.e. for every new selected sample.
In accordance with a characteristic feature of the invention related to this embodiment, the number of input samples is an integral multiple L of the integer,N, with N=2n, characterized in that the predetermined number of samples among the N input samples is an integer Lxc3x97M, with M=2m and 2 less than M less than N, and in that the optimal sample is obtained at the end of a number of average amplitude computations which is equal to M+nxe2x88x92m. By using powers of 2, both for the number of input samples and for the predetermined number of samples, the number of computations necessary for ending the search of the sample is thus much lower and predefined in accordance with a simple relation and is proportional to the logarithm to the base 2 of the sampling operation N.
The invention has the advantage of a very rapid recovery of the optimal sampling instant, which operation can be carried out for every new packet received from anyone of the terminals of the system.
These and other aspects of the invention are apparent from and will be elucidated, by way of non-limitative example, with reference to the embodiments described hereinafter.